I recently bought a new Macbook Pro in China. Two major issues: I can't find my old bookmarks; and Opera now seems optimised for China.

I have the contents of the old computer on an external hard drive. I also have Time Machine from the old computer on an external hard drive.

I want to retrieve my Opera bookmarks but can't find a user Library in either disc drive. Nor will the computer let me update from the old computer's Time Machine.

Quick Search now automatically offers Chinese sites: Baidu, Hao123, 58, 伟大的变革, 天天特惠, as well as Baidu for search. I can't find my old Quick Search options on either hard drive. I also want to switch the search engine to Duckduckgo but it doesn't seem to be an option. (Duckduckgo is available only in the search bar).

Not familiar enough with Macs to answer the first question. The second is a common issue on any platform ... Go to your user settings (on other platforms referred to as your "profile") and find the file "Local state". Leo would recommend running it through a JSON beautifier before editing but I have edited it as is. Open it in a text editor or word processor. Search for the section "location". Obviously yours will say China. You could just delete the entire location section - Opera will then determine your current location on startup. Alternately you could set it to a known good locale - people in Ukraine sometimes prefer to say they are in the U.S. rather than be stuck with Yandex.

Of course if you really wanted you could just delete "Local state" completely, but then you'd lose other settings you've changed.

Have you tried going into System Preferences > Language & Region > Region to check whether it's set there to China?

As for your bookmarks, I don't know where they are stored; there isn't a .plist or a library container which gives them and if they're in the application package, I can't find them there either. One of the devs on the Mac side will have to answer that question, and it might depend on which version of the OS you had when you made those bakups.